Chemiluminescent gas analyzer for measuring the oxides of nitrogen

ABSTRACT

A gas analyzer for determining the concentration of the oxides of nitrogen in a sample gas is provided. The analyzer is particularly adapted for analyzing the exhaust from an internal combustion engine. The analyzer comprises a sample chamber and a reference chamber. An arrangement is provided for delivering sample gas containing the lower oxide of nitrogen (NO) to the sample chamber and a quantity of ozone (O 3 ) for reacting with this oxide of nitrogen and producing a chemiluminescence. After the chemiluminescence is completed, the sample gas is discharged to the reference chamber. A sample photodiode is disposed adjacent to the sample chamber for receiving light emitted from the sample chamber and producing a sample signal representative of the total photoemissivity of the sample gas. A reference photodiode is disposed adjacent to the reference chamber for receiving light emitted from the reference chamber and providing a reference signal representative of the dark current of the photodiodes and the background photoemissivity of the sample gas. A circuit is provided for conditioning and substracting the sample signal and the reference signal to produce an output representative of the concentration of the oxide of nitrogen in the sample gas. Dilution air is mixed with the sample gas either in the instrument with a viscous metering technique or in a sample probe, mounted in the exhaust of the engine, with a sonic metering technique.

BACKGROUND OF THE INVENTION

The invention relates generally to a gas analyzer for determining theconcentration of the oxides of nitrogen in a sample gas and moreparticularly is directed to a chemiluminescence NO, NO₂, NO_(x) analyzerfor determining the concentration of the oxides of nitrogen in theexhaust gas of a combustion engine or power plant.

While there are five oxides of nitrogen, there are only two that are ofprimary concern with regard to the emissions from a combustion engine;namely, NO (nitric oxide) and NO₂ (nitrogen dioxide). The total of theemissions of the oxides of nitrogen is generally referred to as NO_(x).Existing and forthcoming legislative measures both in the U.S. andEurope have created a need for an inexpensive and accurate analyzer formonitoring the oxides of nitrogen emissions of automotive engines.Similarly, there is much interest in monitoring the emissions fromstationary power plants, or the like, and monitoring ambientconcentrations of the oxides of nitrogen. Most of the NO_(x) emitted bygasoline engines is NO which slowly oxides to NO₂. However, in somecombustion engines, such as in a diesel engine where compression ratiosare higher and air fuel ratios are leaner, a substantial amount of NO₂is formed directly during the combustion process.

There are a number of possible techniques for measuring NOconcentrations. These include nondispersive, infrared or ultraviolet gasanalysis. However, infrared gas analysis of NO is difficult because theabsorptivity of NO lies in a range where there is interference withwater vapor. While NO has a very strong ultraviolet absorption linewhere water vapor would not act as a contaminate, nondispersiveultraviolet analysis has not been successful because a very selectivesource of ultraviolet energy is required and the sources which have beendeveloped to date have a very short life span. The most populartechnique for measuring NO in the prior art involves the principle ofchemiluminescence. Chemiluminescence involves the oxidation of NO to N₂instantaneously with O₃ (ozone). When this occurs, the NO₂ which isformed is in an excited state and it immediately returns to its groundstate giving off a photon. The photon emission of the NO₂ returning toits ground state is proportional to the amount of NO in the sample gasas long as stoichiometric or greater quantities of ozone are present.The reaction takes place in approximately 10 milliseconds and forpractical purposes is considered instantaneous. Thus, gas analyzers arefound in the prior art which measure this chemiluminescent reaction witha photomultiplier for the purpose of producing a signal which isrepresentative of the NO concentration in a sample gas.

In fact, the use of chemiluminescent nitric oxide detectors has becomewidespread in the prior art. The typical applications for such detectorsare in air pollution monitoring instruments and gas analyzers fordetermining atmospheric concentrations of the oxides of nitrogen or theconcentrations of the oxides of nitrogen in auto gas emissions, powerplant emissions, etc. The success of prior art chemiluminescentdetectors has almost lead to the adoption of such instruments as defacto legislative standards. However, these prior art chemiluminescentoxides of nitrogen gas analyzers have inherent problems which stem fromthe use of a photomultiplier for measuring the chemiluminescentreaction. Photomultipliers are vacuum tube devices which are large,fragile and expensive. It is generally difficult to supply such a tubewith adequate air flow for cooling and lowering the dark current whileat the same time meeting shielding requirements with regard to ambientlight and radio frequency energy which substantially interfere with theoperation of the device. In some prior art analyzers of this type, athermoelectric cooled photomultiplier tube is used. While this resultsin an instrument having good performance, the cost of the instrument ishigh. Still further, although the gain of a photomultiplier tube ishigh, because the tube comprises a plurality of plates arrayed within aglass envelope, it is difficult to place the detector plates withinclose proximity to the chemiluminescent reaction. Since the photonsissuing from the chemiluminescent reaction are very dispersive anddifficult to focus, this can have a deleterious effect on detectorsensitivity. Other problems with prior art chemiluminescent detectors ingeneral relate to the fact that the sample gas under investigationnormally contains large amounts of CO₂ which has a quenching effect onthe chemiluminescent reaction. The ozone requirements of theseinstruments is relatively high, and ozone is itself a noxious gas. Therange of these instruments can be somewhat limited, and in cases where ahot gas sample is drawn from exhaust of a combustion engine, watercondensate can interfere with the operation of the instrument.

SUMMARY OF THE INVENTION

According to the present invention, these and other problems in theprior art are solved by the provision of a chemiluminescent oxides ofnitrogen gas analyzer featuring a pair of small, inexpensive and durablephotodiodes for measuring the chemiluminescent reaction. According toanother important aspect of the present invention, CO₂ quenching isreduced, the instrument is provided with an extended range, ozonerequirements are reduced and water condensate is substantiallyeliminated by mixing dilution air with the sample gas.

The gas analyzer comprises a sample chamber, an arrangement fordelivering the sample gas of interest to the sample chamber and anarrangement for delivering a sufficient quantity of ozone to the samplechamber for reacting with NO and producing a chemiluminescence. A samplephotodiode is disposed adjacent to the sample chamber for receivinglight emitted from the sample chamber and producing a sample signalrepresentative of the total photoemissivity of the sample gas disposedin the sample chamber. A reference chamber is provided, the referencechamber being disposed immediately adjacent to the sample chamber. Across port is provided between the sample chamber and the referencechamber for discharging the sample gas to the reference chamber afterthe chemiluminescent reaction is completed. A reference photodiode isdisposed adjacent the reference chamber for receiving light emitted fromthe reference chamber and providing a reference signal representative ofboth the dark current of the sample photodiodes and the backgroundphotoemissivity of the sample gas in question. The sample and referencediodes are provided with an isothermal relationship. Processingcircuitry is provided for the photodiodes which includes a samplevoltage follower and a reference voltage follower having high inputimpedances and low current leakage. Thereafter, a differential amplifierdetermines the difference between the output of the sample voltagefollower and the reference voltage follower to provide an accuratemeasure of NO concentration compensated for detector noise or darkcurrent and compensated for the background photoemissivity of the samplegas. In most cases, the NO₂ concentration can be inferred from the NOconcentration and total NO_(x) can be estimated. However, in those caseswhere it is desirable to measure total NO_(x) concentration, a catalystis provided for reducing NO₂ to NO prior to the chemiluminescentreaction. NO concentration can be subtracted from NO_(x) concentrationto provide an accurate measure of NO₂. High-speed low capacitance planardiffusion type photodiodes are used which are small, inexpensive,durable, easily cooled, easily shielded and which feature a planar lightsensitive surface which can be disposed directly adjacent to thechemiluminescent reaction to increase sensitivity.

According to another important aspect of the present invention, anarrangement is provided for introducing dilution air into the sample gasprior to delivery of the sample gas to the sample chamber. In oneembodiment of the invention, the dilution air is introduced in theinstrument with a viscous metering technique to provide a predetermineddilution ratio of air to sample gas. In another embodiment of theinvention, the analyzer is provided with a sample probe, which isadapted for insertion in the exhaust gas of a combustion engine, anddilution air is injected in the sample probe, at the exhaust gaspressure, with a sonic metering technique. In both embodiments of theinvention, dilution of the sample gas substantially eliminates CO₂quenching, extends the range of the instrument, reduces the ozonerequirements of the instrument and reduces problems with watercondensate in the sample gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded assembly of the detector of the gas analyzer ofthe present invention;

FIG. 2 is a plan view partially in section, of the detector of thepresent invention;

FIG. 3(a) is a sectional view of a photodiode used in the gas analyzerof the present invention;

FIG. 3(b) is a condition band model of the photodiode illustrated inFIG. 3(a);

FIG. 4 is a plot of relative sensitive and photoemissivity versuswavelength for a typical photomultiplier, a typical photodiode and thechemiluminescent reaction between NO and O₃ ;

FIG. 5 is a functional diagram of an embodiment of the gas analyzer ofthe present invention wherein dilution of the sample takes place in thegas analyzer;

FIG. 6 is a functional diagram of another embodiment of the gas analyzerof the present invention wherein dilution of the sample gas takes placein the tailpipe of an automotive vehicle.

FIG. 7 is a schematic representation of a voltage following circuit anddifferential amplifier used to process the output of the photodiodes ofthe gas analyzer of the present invention; and

FIG. 8 is a schematic of an instrumentation amplifier, and zero and spanadjustment circuit used to process the output of the photodiodes of thegas analyzer of the present invention and input the same to amicroprocessor controlled display.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to FIGS. 1 and 2 the detector assembly of the gasanalyzer of the present invention is generally illustrated at 10. Thedetector comprises a sample chamber 11 and a reference chamber 12. Ameans for delivering sample gas to the sample chamber 11 is providedcomprising a first concentric tube 14 extending through the back of thesample chamber 11. The sample gas which is delivered to the samplechamber contains an oxide of nitrogen preferably nitric oxide or NO. Ameans for delivering ozone to the sample chamber 11 is also providedcomprising a second concentric tube 15 extending through the back of thesample chamber 11. The second tube 15 provides a quantity of ozone or O₃for reacting with the NO and producing a chemiluminescent reactionwithin the sample chamber 11. Preferably, the reference chamber 12 isdisposed directly adjacent the sample chamber 11 in a common housing 16and a means for discharging sample gas from the sample chamber 11 to thereference chamber 12 is provided comprising a cross port 17. A samplephotodiode 21 is disposed immediately adjacent the sample chamber 11 forreceiving light emitted from the sample chamber and producing a samplesignal representative of the total photoemissivity of the sample gasdisposed in the sample chamber. Similarly, a reference photodiode 22 isdisposed immediately adjacent the reference chamber 12 for receivinglight emitted from the reference chamber and providing a referencesignal representative of both the dark current of the sample photodiode21 and the background photoemissivity of the sample gas contained withinthe reference chamber. The photodiodes 21 and 22 receive light throughthe front face of the sample and reference chambers 11 and 12,respectively, and are mounted in windows 25 and 26 extending throughgaskets 28 and 29 and heat sink 30. The reference and sample photodiodes21 and 22 are mounted in common heat sink 30 to cool the diodes andprovide them with an isothermal relationship. A circuit, illustrated inFIG. 7, is provided for subtracting the output of the referencephotodiode 22 from the output of the sample photodiode 21 to produce asignal representative of the concentration of NO in the sample gascompensated for detector noise or dark current and the backgroundphotoemissivity of the sample gas.

In the prior art, it was thought that photodiodes were not suitablephotodetectors for monitoring a chemiluminescent reaction such as theone between NO and O₃. The photodiodes preferred in the presentembodiment of the invention are a low capacitance, planar diffusion typeknown as the S1337 series of photodiodes available from the HamamatsuCompany of Japan. With reference now also to FIGS. 3(a) and 3(b), thecross section of the photodiodes is illustrated and the dark conditionband model of the photodiodes is given. The low capacitance, planardiffusion type diode preferred for the present application is a highspeed version of the typical planar diffusion type device which makesuse of a highly pure, high resistance N-type material to enlarge thedepletion layer and thereby increase the junction capacitance thuslowering response time to approximately one-tenth the normal value. TheP-layer is also made thin to improve ultraviolet response. Since thedevice is in thermal equilibrium P-layer and N-layer Fermi levels areequal and a voltage gradient develops in the depletion layer by virtueof the contact potential (potential barrier).

Photodiodes were thought to be unsuitable in the prior art because theoutput of a photodiode is at least three orders of magnitude lower thanthat of a typical photomultiplier. That is to say, the typicalphotomultiplier provides a gain approximately one thousand times that ofthe typical photodiode. However, with reference to FIG. 4, which is aplot of relative sensitivity and photoemissivity versus wavelength innanometers, curve 35 illustrates that the photoemissivity of thechemiluminescent reaction of NO and O₃ extends from about 600 nanometersto about 1500 nanometers, while the sensitivity of the typicalphotomultiplier illustrated by curve 36 drops off significantly above900 nanometers. On the other hand the sensitivity of the typicalphotodiode, illustrated at 37, starts just below 600 nanometers andextends beyond 1200 nanometers. While the sensitivities plotted in FIG.4 are not representative of true output since the output of the typicalphotomultiplier is three orders of magnitude higher than the output ofthe typical photodiode, this plot of relative sensitivities issignificant since it shows that the typical photodiode plotted at 37 hasa sensitivity that more closely matches the photoemissivity of thechemiluminescent reaction plotted at 35 than that of the typicalphotomultiplier plotted at 36. However, it has never been thoughtpossible to use photodiodes in this application before because of theirlow output and high dark current. Stated otherwise, the typicalphotodiode provides a low output and a poor signal to noise ratio.Nevertheless, according to the present invention, these problems aresolved by providing two photodiodes in an isothermal relationship. Asample photodiode is provided for monitoring the total photoemissivityof the sample gas, including that due to chemiluminescence and areference photodiode is provided to measure the backgroundphotoemissivity of the sample gas, as well as the dark current in thephotodiodes. Operation of the photodiode pair in this manner providesgood common mode rejection and a much improved signal to noise ratio.Also, as will hereinafter be described, good sample chamber designplaces the planar light sensitive surface of the photodiodes directlyadjacent a planar chemiluminescent display which enhances sensitivityand the provision of high impedance, low leakage current voltagefollowing circuits enables the processing circuit to "see" the lowoutput of the photodiodes.

With specific reference again to FIGS. 1 and 2, it is illustrated thatsample gas and ozone are delivered to the sample chamber 11 throughconcentric inner and outer tubes 14 and 15 which extend in a directiongenerally perpendicular to a glass window 40 which forms the front ofthe sample chamber 11. The concentric inner and outer tubes 14 and 15terminate or are provided with ends which nearly abut the inside surfaceof the window 40. In fact, the ends of the concentric tubes 14 and 15are separated from the inside surface of the window 40 by a distance Dof approximately 0.020 inches. This arrangement and this separationdistance is important to the operation of the detector since, as bestillustrated by the arrows disposed within the sample cell 11 in FIG. 2,ozone and sample gas containing NO are sprayed directly on the insidesurface of the glass window 40 by this arrangement to provide a planarand generally circular area of chemiluminescence which is directed onthe inside surface of the glass 40. This area of chemiluminescence isthus positioned directly adjacent and parallel to the planarphotosensitive surface 41 of the sample detector 21. Since the photonsemitted by the chemiluminescent reaction are very dispersive, thisinsures the best possible response from the sample photodiode 21 and, infact, results in a configuration much better than that possible with thetypical prior art photomultiplier where it was difficult to position thedetector plates of the photomultiplier directly adjacent the area ofchemiluminesence because of the physical dimensions of the evacuatedglass envelope within which the detector plates are enclosed.

Since the chemiluminescent reaction taking place within the samplechamber 11 is practically instantaneous, (on the order of 10milliseconds) by the time the sample gas is discharged into thereference chamber 12 through the cross port 17, (at a typical sampleflow of 2 liters/minute) the chemiluminescent reaction is terminated.The cross port 17 is provided with an angular orientation relative tothe front of the sample chamber 11 and the reference chamber 12 so thatlight generated in the front of the sample chamber 11 from thechemiluminescent reaction does not have a clear path to thephotosensitive surface 42 of the reference diode 22. Still further, theangular orientation of the cross port 17 directs the sample gasexhausted from the reference cell 11 to the inside surface of the glasswindow 40 of reference cell 12. Because the reference and samplephotodiodes are disposed in an isothermal relationship on common heatsink 30, the reference photodiode provides an accurate measure of thedark current within the sample photodiode. However, also important tothe design of the detector assembly is the fact that the referencephotodiode 22 provides a measure of the background photoemissivity ofthe sample gas. It should be appreciated that this is important in caseswhere the analyzer is used to monitor the hot exhaust gas of an internalcombustion engine since such an exhaust gas stream will contain hotphotoemissive particles which will add to the total output of thephotodetector and in effect degrade the signal to noise ratio of theinstrument. However, the reference chamber 12 and reference photodiode22 of the present invention provide an accurate measure of thisbackground photoemissivity and provide a technique for subtracting thesame from the output of the sample photodiode which contains a signalrepresentative of the photoemissivity of the chemiluminescent reactiontaking place in the detector. This is another feature consideredimportant in the construction of a chemiluminescent oxides of nitrogengas analyzer employing photodiodes. After a suitable residence timewithin the reference chamber 12, the sample gas is exhausted throughline 44, extending from the back of the sample chamber 12.

As best illustrated in FIG. 1, the detector assembly comprises analuminum housing 16 within which the sample and reference cells 11 and12 are formed. The surfaces of the cells 11 and 12 may be painted orotherwise suitably coated with a reflective material for directing theoutput of the photoemissive gases contained therein toward the glasswindow 40 which forms the front surface of the cells. Manufacture of thecells in a common block 16 is preferred since it is desirable tominimize any temperature differential between the sample cell 11 and thereference cell 12. The tubes 14, 15 and 44 are threadably secured orotherwise suitably secured in the back of the aluminum block 16. Theglass window 40 is secured to the front face of the detector housing orblock 16 between Viton gaskets 28 and 29 and heat sink 30. Heat sink 30is suitably clamped to the housing 16 with fasteners 48 or the like.Viton is a trademark of the E. I. Dupont Demour Company and identifies afluorocarbon, elastomer material which is opaque, heat insulative andhighly resistant to corrosion. These characteristics are important tothe design of the detector assembly since all ambient light must beexcluded from the reference and detector cells, the sample gas may bequite hot and the corrosive effects of ozone can destroy other gasketmaterials.

With particular reference now to FIG. 2, it is also illustrated that thegasket 28 disposed between the glass window 40 and the heat sink 30, isused to establish a small air gap 50 between the outside surface of theglass 40 and the photodiodes 21 and 22. This air gap is consideredimportant to thermally isolate the photodiodes 21 and 22 from the hotsample gas contained within the reference and sample chambers. This,combined with the heat sink 30, lowers the operating temperature of thephotodiodes 21 and 22. This is important, since heat has a deleteriouseffect on the operation and life of the photodiodes. It should bereadily appreciated that the application of photodiqdes to a gasanalyzer detector of the present type is a significant advance in theart since these diodes are relatively tough, impact resistant deviceswhich are extremely inexpensive when compared to the typical vacuum tubetype photomultiplier and which provide minimal cooling, light shieldingand RF shielding requirements when compared to the typicalphotomultiplier. Still further, the photodiodes may be physicallyoriented relative to the photoemissive chemiluminescent display toenhance the output of the detector.

With reference now to FIG. 5, a functional diagram of the oxides ofnitrogen gas analyzer of the present invention is generally illustratedat 55. In this case, an auto gas analyzer is illustrated, the tail pipeof the combustion engine of the automotive vehicle being schematicallyillustrated at 56. With respect to the previous description, likecomponents are given the same numeral designation and the sample cellhousing is illustrated at 16, the sample cell is at 11, and thereference cell is at 12. The lines 14 and 15 are illustrated enteringthe back of the sample cell 11 while the exhaust line 44 is illustratedleaving the back of the reference cell 12. The sample photodiode isillustrated at 21 and the reference photodiode is illustrated at 22. Theanalyzer includes a sample probe 60 which is adapted for placementwithin the exhaust pipe 56 of the combustion engine. In the case of anauto gas analyzer, the exhaust gas is often quite hot. Thus, the firstconcern relates to the cooling of the sample drawn from the exhaust pipe56. According to the present invention, the hot sample gas drawn fromthe sample probe 60 passes through a line heat exchanger 62 comprising apair of inner and outer concentric flexible lines 63 and 64. Generally,the flexible lines 63 and 64 are approximately 20 feet long and areformed from a flexible Teflon, polymeric material which is capable ofwithstanding relatively high temperatures. The effluent from theanalyzer 55 is pumped back into the tail pipe 56 through the outer tube64 in a counterflow fashion, against the sample which is drawn from theexhaust pipe 56 through the centerline 63. This, in effect, forms acounterflow line heat exchanger which sufficiently lowers thetemperature of the sample gas to permit the use of flexible polymericmaterials for the line 62 which interconnects the sample probe and theanalyzer. This, of course, greatly facilitates the use of the analyzer,permitting flexibility in the placement of the tail pipe mounted sampleprobe and lowering the cost of the flexible connection extending betweenthe tail pipe and the analyzer. The effluent from the analyzer which ispumped back to the tail pipe 56 through the outer flexible line 64 exitsthe sample probe 60 at a point 65 which is downstream from the inlet 66of the sample probe.

A sample pump head disposed at 70 draws sample gas from the sample probe60 through interior line 63 of line heat exchanger 62 to a stainlesssteel heat exchanger 71. The heat exchanger schematically illustrated at71, comprises a length of stainless steel tubing having fins extendingfrom the surface thereof which are cooled by a fan disposed within theanalyzer housing. Since the exhaust gas from an internal combustionengine contains large amounts of water vapor, the gas leaving the heatexchanger 71 is normally saturated and contains a considerable amount ofcondensate. Thus, the sample exiting heat exchanger 71 is inputted to alow volume filter bowl and water trap at 72. The filter bowl and watertrap 72 contains a sintered Teflon filter media which removesparticulate material from the sample flow and the fluid collecting bowlon the bottom of the filter housing is pumped clear by a slow movingperistaltic pump 73. The peristaltic pump 73 returns the condensate vialine 74 to the exterior line 64 of the line heat exchanger 62 fordischarge with the effluent from the analyzer at 65 on sample probe 60.

The pressure of the sample pump head 70 is regulated by a sampleregulator valve 76 which is disposed just upstream from the sample pumphead 70 for introducing a predetermined amount of air to the sample pumphead 70 and establishing a predetermined sample gas pressure. Forexample, in the present embodiment of the invention, the sampleregulator valve 76 is normally adjusted to provide a sample gas pressureon line 78 of approximately 5 inches of mercury. The sample pump head 70and sample regulator valve 76 thus determine the pressure at whichsample gas is drawn from tailpipe 56 and in effect determine thedelivery pressure of sample to the sample chamber 11. The output of thesample pump head 70, which contains a mixture of ambient air and cooledsample gas is directed via line 79 back to the exterior tube of lineheat exchanger 62 for discharge with the remainder of the effluent ofthe analyzer at 65 on sample probe 60.

The analyzer further comprises an effluent pump head 80 for drawingeffluent from reference chamber 12 and a effluent regulator valve isdisposed at 81 for introducing air to the effluent pump head 80 and thusestablishing a predetermined effluent gas pressure. In this case, theeffluent regulator valve 81 is adjusted to provide an effluent gaspressure of approximately 8 inches of mercury in line 82. The output ofeffluent pump head 80, which comprises a mixture of cooled sample gasand air is directed via line 85 back to the exterior line 64 of lineheat exchanger 62 for discharge with the remainder of the effluent ofthe gas analyzer at the sample probe 60. As schematically illustrated bythe line 83, the pump heads 70 and 80 may be driven by a common motor.The total amount of sample gas drawn from the tailpipe 56 of the engineis approximately five liters per minute. A mixture of cooled sample gasand air is pumped back into the sample probe 60 through line heatexchanger 62 at a rate of approximately 15 liters per minute. Thus, theheat exchange capability of the concentric line heat exchanger 62 issubstantial.

The sample gas drawn from an internal combustion engine normallycontains significant amounts of carbon dioxide CO₂ which has a quenchingeffect on the chemiluminescent reaction of NO and O₃. Also, aftercooling the sample gas is normally saturated with water so thatcondensate in the analyzer is a constant problem. Thus, according to thepresent invention, these and other problems in the prior art are solvedby providing an arrangement for introducing dilution air into the samplegas prior to delivery of the sample gas to the sample chamber 11. In thepresent case, a dilution ratio of air to sample gas of approximately 9:1or a turndown of approximately 9:1 is preferred. In addition tosubstantially eliminating problems with quenching and condensate,dilution ratios of 9:1 provide an analyzer with greater range and reduceozone requirements. The latter is particularly important because ozoneis itself a noxious gas.

In the embodiment of FIG. 5, dilution or turndown is accomplished with aviscous metering technique. More particularly, sample gas at the samplegas pressure determined by sample regulator valve 76 is delivered to thenormally closed ports of first and second dilution solenoid valves SVland SV2 via lines 90, 91 and 92. Similarly, dilution air at the samplegas pressure is delivered from regulator valve 76 via lines 94, 95 and96 to the normally open ports of first and second dilution solenoidvalves SVl and SV2. Both the valves SVl and SV2 are provided with anormally open port, a normally closed port and a common port. The commonports are connected together at 98 and form an output which is directedto the sample chamber 11. The first dilution valve SVl is provided withfirst and second flow resistances 101 and 102, which are connected tothe normally closed port and normally open port, respectively, of valveSVl. The first and second flow resistances 101 and 102 are provided witha first predetermined flow resistance value. Similarly, the normallyclosed port and normally open port of valve SV2 are provided with thirdand fourth flow resistances 103 and 104, respectively. The third andfourth flow resistances 103 and 104 are provided with a secondpredetermined flow resistance value. The flow resistances 101 through104 each comprise a capillary tube type of.flow resistance which providea viscous metering effect. In such devices, the flow therethrough isdetermined by the diameter of the capillary and its length. The flowresistances 101 and 102 are sized to permit a total flow ofapproximately 1800 cubic centimeters (cc's) per minute. The third andfourth flow resistances 103 and 104 are sized to permit a total flow ofapproximately 200 cc's per minute. Thus, when sample gas flow througheither of the third or fourth flow resistances 103 or 104 is mixed withdilution air flow through either one of the first and second flowresistances 101 and 102 at common point 98, between solenoid valves SVland SV2, a turndown or dilution ratio of approximately 9:1 is achieved.Within certain viscosity limits, the effects of which are negligible inthis case, this turndown ratio is constant and is determined by thedimensions of flow resistances 101 through 104.

The dilution solenoid valves SVl and SV2 are operated to provide a zerocondition in the analyzer when neither of the dilution valves areactuated and only air is supplied to common point 98 through the flowresistances 102 and 104 and the normally open ports of valves SVl andSV2. A low range for the analyzer is established when both of thedilution valves SVl and SV2 are actuated and only sample gas is suppliedto the common point 98 disposed therebetween from flow resistances 101and 103 connected to the normally closed ports of valves SVl and SV2. Ahigh range is established for the analyzer when only one of the dilutionvalves SVl and SV2 is actuated, such as the valve SV2 inputtingapproximately 1800 cc's per minute of dilution air to common point 98through flow resistance 104 and inputting approximately 200 cc's ofsample gas to common point 98 through the normally open port of valveSVl and flow resistance 102.

In the case of a gasoline engine, most of the NO_(x) generated by theengine is NO, which readily combines with ozone to create a measurablechemiluminescent reaction. However, in the presence of oxygen, the NOslowly oxidizes to NO₂. In other types of internal combustion enginessuch as diesel engines where air/fuel ratios are lower and the enginesoperate at higher compression ratios and temperatures, a certainpercentage of NO₂ is formed directly in the combustion process. In thepresent analyzer it is possible to simply measure the existing NO in thesample and then infer the NO₂ content knowing the residence time of thesample within the analyzer and/or the expected percentage of N0₂ outputof the engine which, in the case of a diesel engine, is approximately 10percent of the total NO_(x) output. However, where it is desirable toprovide a direct measure of total NO_(x) rather than discrete NO, aheated molebdemum or stainless steel gauze catalyst chamber is providedat 110. The catalyst contained within the chamber 110 is heated to anelevated temperature of approximately 600 degrees Celsius. and in thisenvironment, NO₂ is reduced to NO. To facilitate this process, thecommon point 98 between dilution solenoid valves SV1 and SV2 isconnected to the common port of a third solenoid valve or divertor valveSV3. The normally open port of valve SV3 is connected to line 111 whichleads to the interior tube 14 of the concentric tube arrangementinjecting sample gas to the sample chamber 11. The normally closed portof valve SV3 is connected to the catalyst chamber 110, whereby uponactuation of the solenoid valve SV3, sample gas is diverted through thecatalyst chamber 110 to reduce any NO₂ in the sample gas stream to NO sothat the chemiluminescent reaction taking place in sample gas chamber 11is representative of the total NO_(x) in the sample. By switching themode of the analyzer from discrete NO and total NO_(x) with the divertorvalve SV3, an accurate measure of NO₂ can be obtained by subtracting theconcentration of NO from the concentration of NO_(x). Normally this isaccomplished with software in a microprocessor which drives the analyzerdisplay.

Ozone for the chemiluminescent reaction in sample chamber 11 is suppliedthrough a silica gel drier 120, ozone generator 121 and an ozonecapillary type flow resistance 123. Ozone is generated in the ozonegenerator 121 via a corona discharge created by a pair of electrodesacross which an AC voltage is applied. In this case, a glass tube 125 isprovided having a central electrode 126 extending therethrough and anexterior electrode 127 deposited, wound or otherwise suitably formed onthe exterior of the glass tube. A suitable source of AC voltage 128 isimpressed across the electrodes 126 and 127. Air is drawn axiallythrough the glass tube 125 by the reduced pressure within sample chamber11. Silica gel drier 120 is provided for reducing the humidity of theair entering the ozone generator 121 since high humidity can inhibit thecorona discharge and the formation of ozone. The capillary flowresistance 123 is sized so as to provide a somewhat greater thanstoichiometric concentration of ozone to the chemiluminescent reactiontaking place in the sample chamber 11. As previously noted, theprovision of a turndown of 9:1 has two additional beneficial effects onthe operation of the analyzer, one of which is the provision of ananalyzer having extending range, the other of which is a ten-foldreduction in ozone requirements which is important, since ozone isitself a noxious gas.

The gas analyzer further comprises a calibration solenoid valve SV4disposed between the filter bowl 72 and the line 90 which suppliessample gas to the dilution valves SV1 and SV2. The calibration valve SV4includes a normally open port, which is connected to filter 72 and acommon port which is connected to line 90. The normally closed port ofvalve SV4 is connected to a source of calibration gas at 130 throughcontrol valve 131. The normally closed port of valve SV4 is alsoconnected to a T-point 133 on line 78, which is regulated to the samplegas pressure by sample regulator valve 76 so that when the valve SV4 isactuated, the calibration gas, having a known predetermined quantity ofNO, is supplied through valve SV4 at the sample gas pressure determinedby regulator valve 76.

With reference now to FIG. 6, another embodiment of the gas analyzer ofthe present invention is illustrated wherein dilution of the sample gastakes place in the tailpipe 56 of the vehicle. As previously discussed,dilution is considered important to reduce CO₂ quenching, to extend therange of the instrument, to reduce ozone requirements and to reduceproblems with condensate in the analyzer. In the embodiment of FIG. 6,dilution air is pumped into the sample probe 60 where it is introducedto the sample flow with a sonic metering technique. Many components inthis embodiment are similar to those in the embodiment of FIG. 5 andlike components are given the same numeral designation. In thisembodiment of the invention, the line heat exchanger 62 comprises innerand outer concentric, flexible Teflon lines 63 and 64 as before.However, in this case, the outer line 64 carries dilution air ratherthan effluent from the analyzer. Dilution air is pumped to a point 150on sample probe 60 where it is discharged to the atmosphere adjacent tothe tailpipe 56. The point 150 on sample probe 60 is downstream of thepoint 66 on sample probe 60 (which is preferably inside the tailpipe 56)where sample gas or exhaust gas is drawn into the sample probe. Thesample probe 60 includes a filter 151 for filtering particulate matterfrom the sample gas flow. Flow from the filter 151 is then directed to asample sonic orifice 155 while dilution air from point 150 is directedto a dilution air sonic orifice 156. The orifices 155 and 156 are sizedsuch that the pressure drop across the orifices is on the order of 15inches of mercury or more so that sonic flow is achieved in bothorifices. With sonic flow conditions in an orifice, fluctuations indownstream pressure will not effect the mass flow rate through theorifice. Within practical limits, only an increase in upstream pressurewill increase the flow through the orifice. The upstream pressure on thesample sonic orifice 155 is the resultant of the exhaust gas pressureand the pressure drop through the filter 151. The upstream pressure onthe dilution air sonic orifice is atmospheric pressure and is regardedto be a constant. The sample sonic orifice 155 and the dilution airorifice 156 are small, sapphire orifices which are provided withdiameters which are proportionate to and determine the dilution ratio ofair to sample gas. The flow from the sample gas orifice 155 and thedilution air orifice 156 are combined at common point 162 whichdischarges the diluted sample gas flow to the interior line 63 of lineheat exchanger 62. Dilution of the sample gas in the tailpipe eliminatesmuch of the complexity and cost of the embodiment of the inventionillustrated in FIG. 5 in that components such as stainless steel heatexchanger 71, filter bowl and water trap 72, condensate pump 73,dilution valves SV1 and SV2, etc., are eliminated.

Dilution air is supplied to the outer concentric line 64 of line heatexchanger 62 by a simple aquarium type dilution pump 170 which drawsatmospheric air through a filter 152. The output of the pump 170 isregulated by a back pressure dilution regulator valve 171. The output ofthe dilution pump 170 is directed to the line heat exchanger 62 via line172, which includes a control valve 173. Since the dilution air isexhausted to the atmosphere at point 150, dilution air is supplied tothe dilution air orifice at essentially atmospheric pressure. This isregarded as a constant. A pressure sensor 175 is required to provide ameasure of the exhaust gas pressure at a point 178 upstream of the sonicorifice 155. Signals from the pressure transducer 175 are inputted to amicroprocessor 180 which includes a lookup table or a curve fit formodifying the actual dilution ratio and flow as it is affected bychanges in the pressure of the exhaust gas.

Diluted sample gas is drawn through the interior tube 63 of line heatexchanger 62 and into the analyzer by a sample pump 181. The sample gaspressure is determined by a regulator valve 182 disposed on the input ofsample pump 181. In this case, the output of sample pump 181 is simplydischarged to the atmosphere. As in the previous embodiment of theinvention, diluted sample gas is drawn through a divertor valve SV3which, when energized, directs diluted sample flow through catalystchamber 110 for reducing any NO₂ in the sample gas. Diluted sample gasalso flows through first calibration valve SV4. The first calibrationvalve SV4 includes a normally open port connected to the line heatexchanger via line 190. The common port of valve SV4 directs sample to asecond calibration valve SV5. The normally closed port of firstcalibration valve SV4 is connected to a source of calibration gas 130.When the first calibration valve SV4 is actuated, a source of sample gashaving a known concentration of NO is inputted to the sample chamber 11.The second calibration valve SV5 includes a normally open port which isconnected to the common port of first calibration valve SV4 forreceiving sample flow therethrough. The common port of secondcalibration valve SV5 is connected to the normally open port of divertorvalve SV3. The normally closed port of second calibration valve SV5 isconnected to a source of air so that when the second calibration valveSV5 is energized, pure air is inputted to the sample chamber 11 to zerothe analyzer.

The viscous metering technique illustrated in FIG. 5 and the sonicmetering technique illustrated in FIG. 6 produce another importantfeature of the gas analyzer of the present invention. In both cases,these metering techniques provide a means for insuring a constant flowof sample gas through the instrument which is of course importantbecause the intensity of the chemiluminescent reaction which ismonitored is affected by flow rates.

With reference now to FIG. 7, a circuit for processing the output of thesample photodetector and the reference photodetector is schematicallyillustrated. The circuit of FIG. 7 is considered a preamplificationcircuit which is normally mounted directly adjacent the heat sink 30 andphotodiodes 21 and 22. The sample and reference diodes 21 and 22 areoperated in a reverse bias configuration. The output of the sample andreference photodiodes are inputted to unity gain voltage followingamplifiers 201 and 202, respectively. The voltage followers 201 and 202are mounted in a common package and preferably comprise a NationalSemiconductor LF412CN integrated circuit having an industrialtemperature range. This voltage following circuit is desirable becauseit has field effect transistor (FET) inputs which have very highimpedances and very low leakage currents which do little to color thereaction of the photodiodes. It is preferable to employ voltagefollowing circuits 201 and 202 on a common integrated circuit to insurethat the operating conditions of both circuits are identical. Thisinsures good common mode rejection. Power to this circuit is plus andminus 15 volts DC where indicated. The outputs of voltage followingcircuits 201 and 202 are inputted to a unity gain differential amplifier204. Preferably, the differential amplifier 204 is a NationalSemiconductor LM11CN integrated circuit connected to a plus 15 volt andminus 15 volt DC power supply, as indicated. Also, it is preferable thatthe differential amplifier 204 have FET inputs. The output of thedifferential amplifier 204 is representative of the output of samplephotodiode 21 minus the output of reference photodiode 22. The samplephotodiode 21 provides an output representative of the photoemissivityof any chemiluminescent reaction, the background photoemissivity of thesample gas, as well as noise or a dark current. The reference photodiodeprovides a signal representative of the background photoemissivity ofthe sample gas and the dark current. A potentiometer at 205 is providedbetween the output of the reference voltage follower 202 and thedifferential amplifier 204 for adjusting the common mode rejection ratioof this circuit.

Although the output of the preamplifier of FIG. 7 could be inputteddirectly to an analog to digital (A/D) converter, and then to amicroprocessor, in the preferred embodiment of the invention the outputof FIG. 7 is inputted to an amplification and zero offsetting circuitillustrated in FIG. 8. The output of FIG. 7 is first inputted to atypical instrumentation amplifier comprising the three operationalamplifiers U1, U2 and U4. More particularly, the output of FIG. 7 isinputted on pin 3 of operational amplifier U2 or the noninverting inputof the instrumentation amplifier. The inverting input to theinstrumentation amplifier or pin 3 of U1 is connected to ground and isleft floating. This may provide some noise rejection capability. Theinstrumentation amplifier comprising operational amplifiers U1, U2 andU4 preferably comprise a National Semiconductor LM11CN integratedcircuit having a gain of approximately 4000.

The output of the instrumentation amplifier is inputted to an offsetamplifier U5. The offset amplifier U5 is a variable gain amplifier whichcan be adjusted with a span potentiometer 209 in the feedback loop ofthe amplifier. A zero offset value is also inputted to amplifier U5 froma zero offset amplifier U7. The amplifier U7 is a noninverting amplifierwith a gain of two which adds a constant to the output of theinstrumentation amplifier to determine zero. When a calibrated gas isinputted to the instrument, a span adjustment is similarly accomplishedwith the span potentiometer in the feedback loop of the offset amplifierU5. The output of the offset amplifier U5 is inputted to an A/Dconverter 210 and then to a microprocessor 180 which drives a display220. The microprocessor 180 may include software which calculatesinferred NO₂ quantities from discrete measurements of NO and NO_(x) mayinclude curve fitting software to further compensate for temperature andpressure.

The above description should be considered exemplary and that of thepreferred embodiment only. Modifications of the invention will occur tothose who make and use the invention. It is desired to include withinthe scope of the present invention all such modifications of theinvention that come within the proper scope of the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows.
 1. A gas analyzer fordetermining the concentration of an oxide of nitrogen in a sample gascomprising:a sample chamber; means for delivering sample gas to saidsample chamber, said sample gas containing an oxide of nitrogen; meansfor delivering ozone to said sample chamber for reacting with said oxideof nitrogen and producing a chemiluminescence; a reference chamber;means for discharging said sample gas from said sample chamber to saidreference chamber; a sample photodiode disposed adjacent said samplechamber for receiving light emitted from said sample chamber andproducing a sample signal representative of the photoemissivity of saidsample gas disposed in said sample chamber; a reference photodiodedisposed adjacent said reference chamber for receiving light emittedfrom said reference chamber and providing a reference signalrepresentative of the dark current of said sample photodiode and thebackground photoemissivity of said sample gas; and means for subtractingsaid sample signal and said reference signal to produce an outputrepresentative of the concentration of said oxide of nitrogen in saidsample gas.
 2. The gas analyzer of claim 1 further comprising means forintroducing dilution air into said sample gas prior to delivery of saidsample gas to said sample chamber.
 3. The gas analyzer of claim 1further comprising means for providing a constant flow of said samplegas.
 4. The gas analyzer of claim 1 wherein said sample and referencephotodiodes each comprise a low capacitance planar diffusion typephotodiode.
 5. The gas analyzer of claim 1 wherein said samplephotodiode and said reference photodiode are mounted on a common heatsink to provide an isothermal relationship.
 6. The gas analyzer of claim5 further comprising means for diluting the sample gas with air, saidmeans for diluting providing a dilution ratio of air to sample gas ofapproximately 9:1
 7. The gas analyzer of claim 5 wherein a source of hotsample gas is analyzed and a sample gas line heat exchanger is providedcomprising a pair of inner and outer concentric flexible lines extendingfrom said source of hot sample gas to said analyzer, said sample gaspassing through said inner line and effluent from said reference chamberpassing through said outer line.
 8. The gas analyzer of claim 5 whereina source of hot sample gas is analyzed and a sample gas line heatexchanger is provided comprising a pair of inner and outer concentricflexible lines extending from said source of hot sample gas to saidanalyzer, said dilution air being introduced at said source of hotsample gas, said sample gas passing through said inner line and saiddilution air passing through said outer line.
 9. The gas analyzer ofclaim 1 wherein said sample chamber and said reference chamber comprisea pair of cavities disposed in a common cell housing.
 10. The gasanalyzer of claim 9 wherein said reference chamber further includes aplanar light transmitting reference window.
 11. The gas analyzer ofclaim 10 wherein said reference photodiode is provided with a planarlight sensitive surface which is mounted parallel and adjacent to saidreference window.
 12. The gas analyzer of claim 11 wherein said meansfor discharging said sample gas from said sample chamber comprises across port in said cell housing, said cross port extending between saidsample chamber and said reference chamber.
 13. The gas analyzer of claim12 wherein said cross port enters said reference chamber with an angularorientation that directs sample gas to said reference window, saidreference chamber being provided with an exit port disposed on the backthereof behind said window.
 14. The gas analyzer of claim 11 whereinsaid reference window is clamped between said reference chamber and saidreference diode, a first opaque and heat insulating gasket beingdisposed between said reference chamber and said reference window; and asecond opaque and heat insulating gasket being disposed between saidreference window and said sample diode, said second gasket establishinga small heat insulating air gap between said reference window and saidreference diode.
 15. The gas analyzer of claim 1 wherein said samplechamber further includes a planar light transmitting sample window. 16.The gas analyzer of claim 15 wherein said means for delivering samplegas and said means for delivering ozone to said sample chamber comprisesa pair of inner and outer concentric cell tubes, said cell tubesextending into said sample chamber in a direction perpendicular to saidsample window and said cell tubes having open ends disposed adjacentsaid sample window for discharging sample gas and ozone on said samplewindow, whereby a chemiluminescent reaction is carried out on thesurface of said sample window.
 17. The gas analyzer of claim 16 whereinsaid sample photodiode is provided with a planar light sensitive surfacewhich is mounted parallel and adjacent to said sample window.
 18. Thegas analyzer of claim 17 wherein said sample window is clamped betweensaid sample chamber and said sample diode, a first opaque and heatinsulating gasket being disposed between said sample chamber and saidsample window; and a second opaque and heat insulating gasket beingdisposed between said sample window and said sample diode, said secondgasket establishing a small heat insulating air gap between said samplewindow and said sample diode.
 19. The gas analyzer of claim 16 whereinsaid open ends of said cell tubes are disposed approximately 0.020inches from said sample window.
 20. The gas analyzer of claim 1 furthercomprising a sample probe for collecting a representative sample of asample gas, said sample probe being suitable for placement in theexhaust gas stream of an internal combustion engine for determining theconcentration of an oxide of nitrogen in said exhaust gas.
 21. The gasanalyzer of claim 20 further comprising a sample pump head for drawingsample gas from said sample probe into said analyzer.
 22. The gasanalyzer of claim 21 further comprising a sample regulator valvedisposed between said sample probe and said sample pump head forintroducing air to said sample pump head and establishing apredetermined sample gas pressure
 23. The gas analyzer of claim 22further comprising an effluent pump head for drawing sample gas fromsaid reference chamber.
 24. The gas analyzer of claim 23 furthercomprising an effluent regulator valve disposed between said referencechamber and said effluent pump head for introducing air to said effluentpump head and establishing a predetermined effluent gas pressure
 25. Thegas analyzer of claim 24 further comprising means for introducingdilution air into said sample gas prior to delivery of said sample gasto said sample chamber.
 26. The gas analyzer of claim 25 wherein saidmeans for introducing dilution air comprises:a first dilution valvehaving a normally open port, a normally closed port and a common port; asecond dilution valve having a normally open port, a normally closedport and a common port; said common ports of said first and seconddilution valves supplying gas to said sample chamber; a first flowresistance having a first predetermined value connected to said normallyclosed port of said first dilution valve and to a source of sample gasat said predetermined sample gas pressure; a second flow resistancehaving said first predetermined value connected to said normally openport of said first dilution valve and to a source of dilution air atsaid predetermined sample gas pressure; a third flow resistance having asecond predetermined value connected to said normally closed port ofsaid second dilution valve and to a source of sample gas at saidpredetermined sample gas pressure; and a fourth flow resistance havingsaid second predetermined value connected to said normally open port ofsaid second dilution valve and a source of dilution air at saidpredetermined sample gas pressure; said first and second flowresistances having values proportional to a predetermined dilution ratiowhereby: a zero is established when neither of said dilution valves areactuated and only air is supplied to said sample chamber; a low range isestablished when both of said dilution valves are actuated and onlysample gas is supplied to said sample chamber; a high range isestablished when one of said dilution valves is actuated and sample gasis supplied to said sample chamber through a flow resistance having saidfirst predetermined value and dilution air is supplied to said samplechamber through a flow resistance having said second predeterminedvalue.
 27. The gas analyzer of claim 26 wherein said first, second,third and fourth flow resistances each comprise a viscous meteringcapillary tube.
 28. The gas analyzer of claim 22 further comprising acalibration valve disposed between said sample probe and said sampleregulator valve, said calibration valve having a normally open port influid connection with said sample probe, a normally closed port in fluidconnection with a source of calibration gas and said sample regulatorvalve and a common port in fluid connection with said sample regulatorvalve for delivering gas to said sample chamber at said predeterminedsample gas pressure, whereby upon actuation of said calibration valve,calibration gas is supplied to said sample chamber at said predeterminedsample gas pressure.
 29. The gas analyzer of claim 25 wherein said meansfor introducing dilution air comprises a first flow resistance having afirst predetermined value for delivering sample gas to said samplechamber from a source of sample gas at said predetermined sample gaspressure; and a second flow resistance having a second predeterminedvalue for delivering air to said sample chamber from a source of air atsaid predetermined sample gas pressure, said first and secondpredetermined flow resistance values having a ratio proportional to apredetermined dilution ratio.
 30. The gas analyzer of claim 20 whereinsaid exhaust gas is hot and a sample gas line heat exchanger is providedcomprising a pair of inner and outer concentric flexible lines extendingfrom said sample probe to said analyzer, said sample gas passing throughone of said concentric lines and effluent from said reference chamberpassing through the other of said concentric lines.
 31. The gas analyzerof claim 30 further comprising a stainless steel instrument heatexchanger for receiving sample from said line heat exchanger.
 32. Thegas analyzer of claim 31 further comprising a filter bowl and water trapfor receiving sample from said instrument heat exchanger; and aparastoltic pump for removing condensate from said water trap andreturning the same to said other of said concentric lines.
 33. The gasanalyzer of claim 20 further comprising a divertor valve disposedbetween said sample probe and said sample chamber and a catalyst chamberfor reducing NO₂ to NO, said catalyst chamber being in fluidcommunication with said sample chamber, said divertor valve having anormally open port connected to said sample chamber, a normally closedport connected to said catalyst chamber and a common port connected tosaid sample probe, whereby upon actuation of said divertor valve, samplegas is diverted through said catalyst chamber where NO₂ is reduced to NOand said analyzer provides an output representative of the total NO_(x)concentration in said sample gas.
 34. The gas analyzer of claim 20further comprising means for introducing dilution air into said samplegas prior to delivery of said sample gas to said sample chamber.
 35. Thegas analyzer of claim 34 wherein said means for introducing dilution airfurther comprises means disposed on said sample probe for diluting saidsample gas with air.
 36. The gas analyzer of claim 35 wherein said meansdisposed on said sample probe for diluting comprises:a means forming asample sonic orifice for receiving exhaust gas; a means for forming adilution air sonic orifice for receiving dilution air.
 37. The gasanalyzer of claim 36 wherein said sample probe further comprises apressure transducer for monitoring changes in the exhaust gas pressureand thus providing a measure of the change in an air/sample gas ratiodue to changes in exhaust gas pressure and means for delivering dilutionair to said dilution orifice at atmospheric pressure.
 38. The gasanalyzer of claim 37 wherein said sample orifice and said dilutionorifice are provided with dimensions that are proportional to apredetermined dilution ratio.
 39. The gas analyzer of claim 39 furthercomprising a sample pump for drawing exhaust gas and dilution air fromsaid sample probe through said sample chamber and said referencechamber; and a sample regulator valve disposed on the input of saidsample pump for establishing a predetermined sample gas pressure. 40.The gas analyzer of claim 39 further comprising a calibration valvedisposed between said sample probe and said sample chamber, saidcalibration valve having a normally open port connected to said sampleprobe, a common port connected to said sample chamber and a normallyclosed port connected to a source of calibration gas, whereby actuationof said calibration valve supplies said calibration gas to said samplechamber.
 41. The gas analyzer of claim 37 further comprising a samplegas line heat exchanger comprising a pair of inner and outer concentricflexible lines extending from said sample probe to said analyzer,dilution air traveling to said sample probe through one of saidconcentric lines and a mixture of exhaust gas and dilution air travelingfrom said sample probe to said analyzer through the other of saidconcentric lines.
 42. The gas analyzer of claim 41 further comprising adilution pump for supplying dilution air to said line heat exchanger andsaid sample probe; and a dilution regulator valve disposed on the outputof said dilution pump for providing dilution air at a predeterminedpressure.
 43. The gas analyzer of claim 38 further comprising a aprocessor for receiving pressure signals from said pressure transducerand providing a signal representative of the actual dilution ratioadjusted for the pressure of the exhaust gas.
 44. The gas analyzer ofclaim 41 further comprising a zero valve disposed between said sampleprobe and said sample chamber, said zero valve having a normally openport connected to said sample probe, a common port connected to saidsample chamber and a normally closed port connected to a source of air,whereby actuation of said zero valve supplies air to said samplechamber.
 45. The gas analyzer of claim 1 wherein said means fordelivering ozone to said sample chamber further comprises an air drierand an ozone generator, said ozone generator comprising an air chamberfor receiving air from said drier, a pair of electrodes disposed in saidair chamber, means for applying an AC voltage to said pair of electrodesand an ozone flow resistance disposed between said air chamber and saidsample chamber.
 46. The gas analyzer of claim 1 wherein said ozone flowresistance comprises a viscous metering capillary tube.
 47. The gasanalyzer of claim 1 wherein the output of said sample photodiode andsaid reference photodiode are each inputted to a sample voltage followerand a reference voltage follower, respectively, said voltage followershaving a high input impedance and a low leakage current.
 48. The gasanalyzer of claim 47 further comprising a differential amplifier fordetermining the difference between the output of said sample voltagefollower and said reference voltage follower, said differentialamplifier having a high input impedance and a low leakage current.